1. Field of the Invention
This invention relates to the mechanical arts. In particular, it relates to connectors for fluid flow tubes and methods for making the connectors.
2. Discussion of Relevant Art
The measurement and control of the flow of gases is important in many industries. During the manufacture of semiconductors, for example, many of the processes require a precise reaction of two or more gases under carefully controlled conditions. Since chemical reactions occur on a molecular level, the control of mass flow is the most direct way to regulate the reactants.
There have been developed in the art a variety of instruments for measuring the mass flow rate of gases from below 5 standard cubic centimeters (scc) per minute to more than 500,000 scc/min. The prevalent design of such instruments requires that the flow of the gas be divided into two or more paths. Representative instruments include the flow meters disclosed in U.S. Pat. Nos. 4,524,616 and 3,983,384, which patents are herein incorporated by reference.
In a typical instrument, a small flow is routed through a fluid flow tube located in a flow sensor assembly, where the flow is measured, while most of the flow is routed through a splitter section located in parallel with the flow sensor assembly.
The flow sensor assembly must be easily removable from the flow meter for servicing, yet when in place it must be sealed from the external environment to an extraordinary extent. For example, it is necessary to seal around the end portions of the fluid flow tube to avoid any cross flow between the atmosphere and the process gas. Typically, a leak between the process gas and the atmosphere greater than about 10.sup.-9 scc/sec helium causes a problem of some type.
Because of the small size of the fluid flow tube and the severe limitation on the types of sealing materials available due to the interaction between the sealing material and process gases, a major problem with existing mass flow meters is leakage of the flow sensor assembly. Existing mass flow meters use one of two methods to attach the end portions of the fluid flow tube to the remainder of the flow meter. They either use rubber O-rings alone or they use braze joints between the end portions of the fluid flow tube and washers which then mate with rubber or metal O-rings.
Rubber O-rings are limited in their application, because of their interaction with highly reactive process gases and because they are porous to light gases at leakage levels around 10.sup.-6 scc/sec of helium. Metal O-rings are expensive and normally cannot be reused once the flow sensor assembly has been removed for servicing. Both rubber and metal O-rings form pockets which can trap impurities. Braze joints introduce a new material (braze alloy) into the flow stream which may react with some of the highly reactive process gases used in semiconductor manufacturing.
Attempts have been made to use taper pin connectors to seal flow tubes in mass flow meters. Unfortunately, the attempts have been unsuccessful due to the difficulty of making very small taper pins and the corresponding taper bores with sufficient accuracy and with surfaces having sufficiently smooth and complimentary finishes to form an essentially leak-free connection.